Antistatic shoe sole

ABSTRACT

A shoe sole construction comprises a polyurethane outer sole having an electrical resistance of approximately 40 MOhms in a standard wet/dry test and an electrical resistance of approximately 20 MOhms in a standard wet/wet test in a test conducted according to British Standard 2050, and a rubber sole insert having an electrical resistance of approximately 1 MOhm positioned adjacent the outer sole. With this construction, static electricity in the sole insert is capable of discharging from the sole insert along a path of least resistance through the outer sole.

BACKGROUND OF THE INVENTION

The present invention relates to an antistatic shoe sole.

In the manufacture of some products, for example electronic devices,stray static electricity can damage or significantly disrupt theoperation of the products. Sensitive computers, integrated circuits andcomputer software and data which are stored on magnetic tape or disk mayeven be destroyed by stray static electricity. Consequently, extensivemeasures are taken during the manufacture and use of these products inorder to reduce the risks of damage by stray static electricity. Peopleusing and manufacturing the products are usually one of the mostsignificant sources of stray static electricity. Consequently, it isessential to discharge static electricity from people who may come incontact with products which may be adversely effected by staticelectricity. Usually working areas in which such products can be foundhave a grounded floor or grounded mats which are able to discharge thestatic electricity from people walking on the floor or mats byconducting the static electricity from the people to the floor. The onlycontact people have with the floor is usually through their shoe soles,but static electricity does not readily discharge through conventionalshoe soles since the shoe soles do not adequately conduct staticelectricity from the body of the person to the floor. Therefore, inorder to improve static electricity discharge it is desirable to have anantistatic shoe which readily discharges static electricity.

DISCUSSION OF PRIOR ART

It is known to produce antistatic shoes with rubber soles, but theseshoes suffer from a number of disadvantages. Rubber soles are oftenheavy and may mark floors. In order to overcome these problems, it ispreferably to use polyurethane rather than rubber, since polyurethane isa lighter product which lasts longer and is non-marking. However,polyurethane is not within the desired range of electrical resistancefor conducting static electricity. Antistatic shoes usually have anelectrical resistance property of between 75 kOhms and 50 MOhms, (asmeasured by the standard test specified in British Standard 2050).Polyurethane does not have these resistance properties and it istherefore necessary to enhance the antistatic properties of thepolyurethane by the introduction of additives to the polyurethane.However, polyurethane is a blown material and the dispersion ofadditives within the material is not homogenous due to the poor mixingcharacteristics of polyurethane. Furthermore, in order to achieveadequate antistatic properties it is necessary to add a large amount ofadditives, but this deteriorates the quality of the polyurethane.Consequently, simply adding antistatic additives to polyurethane to makean antistatic polyurethane shoe sole is not possible.

SUMMARY OF THE INVENTION

The present invention attempts to overcome one or more of the aboveproblems.

According to the present invention there is provided a shoe solecomprising:

a polyurethane outer sole having predetermined electrical resistancecharacteristics, and

a rubber sole insert having predetermined electrical resistancecharacteristics positioned adjacent the outer sole,

wherein, static electricity in the sole insert is capable of dischargingfrom the sole insert along a path of least resistance through the outersole.

It is preferred that the shoe sole further comprises a conductive innersole positioned adjacent the sole insert so as to improve the dischargeof static electricity from a wearer of a shoe having the shoe solethrough the inner sole, the sole insert and the outer sole.

DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings, inwhich;

FIG. 1 is a perspective view of a preferred embodiment of the presentinvention,

FIG. 2 is a top view of the sole of FIG. 1, and

FIG. 3 is a preferred embodiment of a shoe having an antistatic sole ofFIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment of the shoe sole 1, as illustrated in FIGS. 1,2 and 3, is comprised of an outer sole 2 and a sole insert 4. The shoesole 1 comprises a heel section 3 and a toe section 5. The sole insert 4is comprised of a strip of rubber which is smaller than the shoe sole,but of a similar shape. The sole insert 4 extends substantially alongthe centre of the outer sole 2, from about the centre of the heelsection 3 towards the toe section 5 of the shoe sole. The sole insert 4is recessed in the outer sole 2 to prevent movement of the sole insert4.

The outer sole 2 is manufactured from a microcellular polyurethanecontaining approximately 5% of quaternary ammonium, or a similarcompound, and 2 to 3% carbon black to impart electrical resistancecharacteristics to the polyurethane material for the necessaryantistatic properties. The additive is supplied by ICI Australia Limitedunder the trade mark "Additive I". This polyurethane sole has aresistance of approximately 40 MOhms in a standard wet/dry test and aresistance of 20 MOhms in a wet/wet test conducted according to BritishStandard 2050. In general, the resistance cannot be reduced any furthersince a maximum of approximately 10% additives can be added topolyurethane before it starts to break down. Therefore, the resistanceof the polyurethane is reduced as far as possible, but since theimproved electrical resistance characteristics are still not adequatefor an antistatic shoe sole, the characteristics have to be improvedwith the aid of a sole insert 4 having a lower electrical resistance.The combination of the outer sole 2 and the sole insert 4 thus providesan antistatic shoe sole with the required electrical resistancecharacteristics.

The sole insert 4 is manufactured from a rubber containing variousadditives which results in a sole insert with a resistance ofapproximately 1 MOhms. Because rubber does not suffer from poor mixing,as polyurethane does, the additives are dispersed evenly through theinsert resulting in even conductivity. Furthermore, the rubber can begiven more additives than polyurethane, therefore resulting in a muchlower resistance value compared to polyurethane. The base formulationfor the antistatic rubber insert is as follows:

    ______________________________________                                        Natural Rubber SMR20                                                                              80.00                                                     Synthetic Rubber SBR1502                                                                          20.00                                                     Zinc Oxide          5.00                                                      Stearic Acid        1.00                                                      Antioxidant TMQ     1.00                                                      Accelerator CBS     1.50                                                      Accelerator DPG     0.25                                                      Carbon Black N220   10.00                                                     Carbon Black N774   55.00                                                     Aromatic Oil        8.00                                                      Sulphur             2.00                                                                          183.75                                                    ______________________________________                                    

Carbon black is a substantially pure form of finely divided carbon.Carbon black N220 is a conductive black and its proportions to carbonblack N774 are adjusted to provide the required electrical resistance.Increasing the amount of carbon black N220, provides a lower resistancevalue for the rubber.

In use, static electricity from a person wearing a shoe having the shoesole of the preferred embodiment of the present invention, collects inthe sole insert 4. In order to discharge to the ground, the staticelectricity finds a path of least resistance through the polyurethaneouter sole 2. This location would usually be near the toe of the shoe,since the shoe sole is at its thinnest point in this area. However, dueto the uneven mixing of the antistatic additives within thepolyurethane, the path of least resistance is not always the shortestpath in distance through the shoe sole, but rather the path which ismost conductive to electricity. If static electricity builds up in theheel 3 of the shoe sole 1, then the static electricity will most likelyflow further down the sole insert 4 and find a discharge path throughthe thinner toe section 5 of the outer sole 2. In the preferredembodiment the thickness of the sole is at least 2 to 3 millimeters.

FIG. 4 illustrates a shoe 10 having an outer sole 2 in accordance with apreferred embodiment of the present invention. The shoe 10 has a leatherupper 6 with a polyurethane outer sole 2, as described above. In orderto improve the conductance of static electricity from a wearer of theshoe to the shoe sole, the shoe is provided with a conductive inner sole9 which is in direct contact with the sole insert 4. The leather outer 6is tucked under the insole 9, as shown at 7 and 8, and continues aroundthe circumference of the shoe sole so that the leather abuts the soleinsert 4. In use, static electricity flows from the body of a wearer ofthe shoe, through the conductive inner sole 9 to the sole insert 4 andthen through the polyurethane outer sole 2 along a path of leastresistance.

The polyurethane outer sole has advantages over the prior art rubbersoles in that the polyurethane soles are non-marking, have betterantislip characteristics, last longer and are lighter to wear.Furthermore, unlike rubber, the polyurethane sole is resistant topetrochemicals and acids, thus making it ideal in hazardousenvironments.

It will be appreciated that features of the above invention may bevaried for different applications the foregone description of theembodiments of the invention have been presented for purposes ofillustration only. It is not intended to be exhaustive or to limit theinvention to the embodiments, and many variations and modifications willbe obvious to one skilled in the art.

We claim:
 1. A shoe sole comprising: a polyurethane outer sole having anelectrical resistance of approximately 40 MOhms in a standard wet/drytest and an electrical resistance of approximately 20 MOhms in astandard wet/wet test conducted according to British Standard 2050, anda rubber sole insert having an electrical resistance of approximately 1MOhm positioned adjacent the outer sole such that static electricity inthe sole insert is capable of discharging from the sole insert along apath of least resistance through the outer sole.
 2. A shoe soleaccording to claim 1, wherein the sole insert comprises a stripextending substantially along the center and on an inner surface of theouter sole.
 3. A shoe sole according to claim 2, wherein the sole insertextends from the heel of the shoe sole to the toe of the shoe sole.
 4. Ashoe sole according to claim 2, wherein the outer sole has a centralrecess in the inner surface thereof and the sole insert is located inthe central recess.
 5. A shoe sole according to claim 1, furthercomprising a conductive inner sole positioned adjacent the sole insert.6. A shoe sole according to claim 1, wherein the outer sole is comprisedof polyurethane having approximately 5% quaternary ammonium at 2 to 3%carbon black.
 7. A shoe sole according to claim 1, wherein the soleinsert is comprised of a rubber material having approximately 5% carbonblack N220.
 8. A shoe sole according to claim 1, wherein the outer solehas a thickness of at least 2 mm.
 9. A shoe including a shoe solecomprising: a polyurethane outer sole having an electrical resistance ofapproximately 40 MOhms in a standard wet/dry test and an electricalresistance of approximately 20 MOhms in a standard wet/wet testconducted according to British Standard 2050, and a rubber sole inserthaving an electrical resistance of approximately 1 MOhm positionedadjacent the outer sole such that static electricity in the sole insertis capable of discharging from the sole insert along a path of leastresistance through the outer sole.